1. Field of the Invention
The present invention relates to the scrambling of intelligence signals, in order to prevent the receipt by unauthorized persons of the intelligence contained in them, and to the unscrambling the scrambled signal, by an authorized recipient, in a manner which reduces the noise introduced into the scrambled signal during the scrambling/unscrambling process.
2. Description of the Related Art
Inversion of selected portions of a signal (that is, multiplication of the signal values of selected portions by -1) is well known as a security measure.
For example, in U.S. Pat. No. 2,402,058 issued to Loughren and herein incorporated by reference, inversion of an audio signal is controlled by a signal developed from a cathode ray tube/code card/photocell arrangement. The code card is placed in front of the CRT and is generally opaque, but has a plurality of randomly placed and shaped apertures which allow the CRT's electron beam to project through the code card, developing an inversion control signal at the output of the photocell. The control signal is random in that the pattern on the code card which generates the inversion control signal is dependent upon the spacing and configuration of the card's apertures, which are themselves random. Both the encoder and decoder employ identical code cards, which are replaceable with ones having different aperture patterns. The control signal has a slightly higher fundamental frequency range than the audio to be encoded, and the control and audio signals are effectively mixed by modulating the audio signal on the control signal, thereby producing upper and lower sidebands. During transmission, only the lower sideband is transmitted, further concealing the original audio signal. However, the use of single sideband transmission relaxes the precision of the control signal on the decoder side. To compensate, Loughren combines one or more constant frequency tones with the audio signal prior to inversion, which are removed at the decoder by a sharply tuned rejector filter. These constant frequencies are best added a only during intervals of active audio, hindering an authorized listener from deducing these added frequency values.
Other types of pseudo-random inversion signal generators are also known in the art. For example, U.S. Pat. No. 3,610,828 issued to Girard et al., herein incorporated by reference, describes a polarity-reversing switch controlled by a code generator which emits a series of binary signals in synchronism with a clock. Based on the value of the code generator, the polarity-reversing switch either inverts the audio signal or passes it unaffected. The code generator is a shift register in which the contents of the last stage control the state of the polarity-reversing switch. The shift register is provided with internal feedback connections to provide a series of pseudo-random output bits in response to the clock pulse. Additionally, a code selector network is included for insuring that both the encoder and the decoder are synchronized: should they not have the identical code in their respective code selector networks, the decoder will not properly decode the encoded signal. For added security, this code could easily and frequently be changed by the user. To help conceal the original audio signal, a d.c. voltage, dependent upon the state of the polarity-reversing switch, is added to the signal prior to its inversion.
There are several problems with inversion scrambling techniques as found in the above references. For example, to help conceal the original audio, signals of either constant amplitude or constant frequency are added to the original audio signal, usually only during intervals of active audio by a switching network. A major disadvantage of using a constant amplitude or constant frequency signal to conceal the original audio signal is that its consistency can easily be detected as background noise when the audio signal is low amplitude or low frequency. Additionally, if the switching network does not cut off the concealing signal exactly at the termination of the audio signal, an authorized listener can readily detect the signal's change of state, thereby isolating the concealing signal's characteristics.
In addition, a problem exists due to the inherent bandwidth limitations of the transmission path. At each inversion, an abrupt transition, mathematically described as the sum of a series of an infinite number of frequencies, each frequency having a different amplitude, occurs. As all transmission paths have a limited bandwidth, some of these higher frequencies will be lost during transmission of the scrambled signal. Accordingly, an artifact will appear at the inversion points of the unscrambled signal.
U.S. Pat. No. 2,987.576 issued to Druz et al., herein incorporated by reference, tries to solve the problem of bandwidth limitation. Druz unscrambles the audio signal and splits the unscrambled audio into two paths prior to output. The first path includes a sampler for sampling the unscrambled audio signal and a low-pass filter network for removing the artifact; the second path contains a delay network so that the outputs of the first and second paths are synchronous. An electronic switch outputs the true unscrambled audio at all points in time except when the artifact, due to inversion, occurs. At the inversion points, the switch outputs the filtered unscrambled audio signal for a predetermined period of time. Thus, at the inversion points, a filtered version of the unscrambled audio signal is output for a predetermined interval. Although the Druz system tries to correct the problems inherent with limited bandwidth transmission paths, the correction signal itself contains undesired distortion caused by noise components which have been demodulated down into the audio range by the sampler. Thus, distortion introduced during the inversion scrambling/unscrambling process is present in the final output audio signal.